Fiber alignment device; method of making and using

ABSTRACT

An optical fiber (1) has an endface (2) which has been coated with a metal oxide coating (5) such that the light guiding core region (4) of the endface (2) is at least partially bounded by the coating (5) while being uncoated itself. The core region (4) has a different reflectivity than that of the boundary region (3). The fiber may be used in a system for aligning a fiber with an optical device (20).

This invention relates to an optical device having an optical waveguide.

It is often necessary to align a passive or active optical device withan optical waveguide, in order to obtain maximum efficiency in couplingoptical signals between the waveguide and the device. When the waveguideis an optical fibre the light guiding region of the fibre, that is, theinner core region, must be aligned with the light emitting or lightreceiving region of the device.

In a known method of aligning an optical fibre with a laser, an opticalsignal from the laser is arranged to be incident on the fibre. Theintensity of the signal emerging from the fibre is measured, and theposition of the fibre relative to the laser is altered in order toobtain maximum intensity of the signal emerging from the fibre. Thealignment is thus set initially by measurement. A disadvantage of thisknown method is that the alignment is set only initially and assumed toremain satisfactory thereafter. However, subsequent movement, vibrationor temperature stresses may result in movement of the laser relative tothe fibre. Any such movement therefore remains undetected, and maximumefficiency in coupling may no longer be achieved.

In a known method of detecting pits which form tracks in a compact disc,the pits are detected by a servo mechanism which detects the pits andadjusts a laser relative to the pits in order that the laser is lockedonto the tracks. The servo mechanism acts to dynamically adjust therelative positions of the laser and the pits in order to maintainmaximum efficiency. The mechanism detects the pits in the compact disc,by detecting a difference in reflectivity between the pits and theremaining surface of the disc. Such servo laser devices are massproduced and therefore relatively cheap optical sources. However, theycannot be used to align the laser with known optical waveguides. It isan object of the present invention to solve this problem.

According to a first aspect of the present invention there is providedan optical apparatus comprising an optical waveguide having an endface;a light guiding region exposed at the endface and bounded at the endfaceby at least two opposing boundary regions, the reflectivity of the lightguiding region being different to that of the boundary regions,characterised in that the apparatus further includes and optical source;alignment means responsive to the difference in reflectivity between thelight guiding region and the boundary regions for aligning the opticalsource with the light guiding region.

An optical device can be arranged to scan the endface of the waveguideand thus detect and locate the position of the light guiding region.This thus enables the mass produced servoed laser devices to be used asoptical sources for waveguide systems although, it is to be understoodthat some modification of the devices used in relation to compact discsmay be necessary. For instance, the coupling arrangement between thelaser and the waveguide may require modification.

According to a second aspect of the present invention there is provideda method of fabricating an optical waveguide having an endface; a lightguiding region exposed at the endface and bounded at the endface by atleast two opposing boundary regions exposed at the end-face, thereflectivity of the light guiding region at the end-face being differentto that of the boundary regions, the method comprising coating at leasta portion of the endface of the waveguide with a material which is ofthe type which breaks down when heated; passing an optical signalthrough the light guiding region thereby producing heat sufficient tocause the material to break down in the light guiding region of theendface but not in the boundary regions.

Conveniently, the waveguide is an optical fibre and the light guidingregion is an inner core region, and the boundary region is an outercladding region.

Alternatively, a planar waveguide such as lithium niobate with atitanium in-diffused channel may be used.

Preferably, the optical fibre comprises a coating of material coveringsubstantially all of the cladding region and is fabricated by coatingsubstantially all of the endface of the waveguide with the materialwhich is of the type which breaks down when heated; passing an opticalsignal through the light guiding region thereby producing heatsufficient to cause the material to break down in the light guidingregion.

Removal of the material in this way results in the automatic alignmentof the surface reflecting boundary to the edge of the light guiding coreregion of the fibre at or near its boundary with the cladding.

The invention will now be described by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a perspective view of a known optical fibre;

FIG. 2 is a perspective view of an optical fibre suitable for use withthe present invention;

FIG. 3 is a longitudinal cross section of the fibre of FIG. 2;

FIG. 4 is a schematic representation of a servo mechanism and fibreforming an alignment system in accordance with the invention; and

FIG. 5 is a perspective view of planar optical waveguide made suitablefor use with the present invention.

Referring to FIG. 1, a known optical fibre 1 comprises an endface 2having an outer boundary region 3, and an inner light guiding coreregion 4. Light travelling along the fibre is guided through the fibrewithin the light guiding region.

Referring to FIGS. 2 and 3, an optical fibre suitable for use with theinvention is shown. The parts of the fibre which correspond to parts ofthe fibre of FIG. 1 are given equivalent reference numerals. The endface2 of the fibre 1 has been coated with a coating 5 such thatsubstantially all of the outer cladding region 3 is covered by thecoating 5, and the inner core region 4 is substantially completelyuncovered by the coating 5.

One method of forming the endface is to form a coating, for example, ametal oxide, and apply it to the endface 2 by evaporating or sputteringthe metal oxide onto the endface. This results in substantially all ofthe endface being coated with the coating. An intense optical beam isthen passed along the fibre 1. The intense optical beam is constrainedby the fibre design to travel in the same spatial volume of the fibre inor near the core that a normal lower power signal would occupy. Thiscauses the coating 5 over the core region 4 of the fibre to be removedby evaporation in the particular area which it is required to align withadjacent devices. The coating 5 remains covering substantially all ofthe cladding region, and has a reflectivity which is different from thatof the core region 4. A metal oxide is chosen to form the coating as itwill have an amorphous structure and will absorb the energy of theintense optical beam sufficiently to burn off. Other types of materialssuch as certain metals may also be used. The endface 2 of the fibre,when viewed axially, now comprises the core region 4 from which thecoating 5 has been removed, surrounded by an annulus of coating 5.Because of the difference in reflectivity of the coating 5 and the core4, an optical detecting device can be used to locate the core 4 by thedifference in reflection from the core 4 and the annular coating 5 andso align the laser with the core.

Another method of forming the endface is to use selective deposition bymasking.

Referring to FIG. 4, an alignment system in accordance with theinvention is shown schematically. The system comprises an optical fibre1 of the type shown in FIGS. 2 and 3 and an optical tracking source 6.The optical tracking source 6 is coupled to a laser 20 for producing anoptical signal. The optical signal is focussed by lens 21 onto a beamsplitter 22, such as a half reflecting mirror. The beam splitter 22divides the optical signal into two beams such that a first portion isincident upon a second, moveable lens 23 which focusses the firstportion onto the fibre endface. The beam splitter 22 directs a secondportion of the optical signal onto a third lens 24 which focusses thesecond portion onto a matrix detector 25. A feed back and control system26 links moveable lens 23 and matrix 25 to align the first portion ofthe optical beam with the waveguiding region of the fibre 1.

Referring to FIG. 5, a planar waveguide made in accordance with theinvention is shown. The waveguide 7 comprises a lithium niobate block 8having a titanium in-diffused channel 7 which is a light guiding region.The endface 9 of the waveguide 7 has been coated with a coating suchthat two opposite boundary regions 11, 12 of the lithium niobate block 8are covered by the coating, and the light guiding region 7 is leftuncovered.

In this context, the term "optical" is intended to refer to that part ofthe electromagnetic spectrum which is generally known as the visibleregion together with those parts of the infrared and ultraviolet regionsat each end of the visible region which are capable of being transmittedby dielectric optical waveguides such as optical fibre.

I claim:
 1. An optical apparatus comprising:an optical waveguide havinga longitudinal axis and an endface transverse to said axis; a lightguiding region exposed at the endface and at least partially bounded atthe endface by at least one boundary region coated onto said endface,the reflectivity of the light guiding region being different from thatof the boundary region, characterised in that the apparatus furthercomprises an optical source, and alignment means, responsive to thedifference in reflectivity between the light guiding region and theboundary region, for aligning the optical source with the light guidingregion.
 2. Apparatus according to claim 1 wherein the optical source isa laser.
 3. Apparatus according to claim 1 in which the light guidingregion of the optical waveguide is an inner core region of an opticalfibre and the boundary region covers at least part of an outer claddingof said optical fibre.
 4. Apparatus according to claim 1 in which theboundary region of the optical waveguide comprises an annular coating.5. Apparatus according to claim 4 wherein the coating is a metal oxide.6. A method of fabricating an optical waveguide having a longitudinalaxis along a light guiding region, bounded at least partially by aboundary region coated onto an endface of said waveguide, said endfacebeing transverse to said axis, the reflectivity of the light guidingregion being different from that of the boundary region, the methodcomprising the steps of:coating at least a portion of said endface ofsaid optical waveguide with a material which is of a type which breaksdown when heated; and forming said boundary region by passing an opticalsignal through the light guiding region of said waveguide therebyproducing heat sufficient to cause the material to break down where itcoats the light guiding region of the endface but not where it formssaid boundary region.
 7. A method according to claim 6 wherein saidendface is coated by evaporating onto the endface.
 8. A method accordingto claim 6 wherein the heat produced by the optical signal causes thematerial to evaporate.
 9. A method of aligning an optical source to alight guiding region of an optical waveguide having a longitudinal axisand an endface transverse to said axis said light guiding region beingat least partially bounded by at least one boundary region coated ontosaid endface, the reflectivity of the light guiding region beingdifferent from that of the boundary region, the method comprising thesteps of:measuring the difference in reflectivity across the endface,thereby detecting the location of the light guiding region; and movingthe optical source relative to the waveguide in response to thereflectivity measurement, thereby aligning the optical source with thelight guiding region.